This invention relates to the chemical vapor deposition of diamond or carbon films, and more particularly, to an apparatus and a method of use in such deposition.
One class of methods developed in recent years for carbon deposition consists of the chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) methods. For a general summary of various deposition methods including CVD methods, reference is made to Chemical and Engineering News, 67(20), 24-39 (May 15, 1989), incorporated herein by reference.
In the CVD methods, a mixture of hydrogen and a hydrocarbon gas such as methane is introduced into a chamber and is then heated or thermally activated. Some of the hydrogen gas is disassociated into atomic hydrogen which reacts with the hydrocarbon to form growth species. These species deposit on the substrate in the form of a carbon film when they come into contact with a cooler substrate. This process is schematically illustrated in FIG. 1.
One of the many of the CVD coating methods, hereinafter referred to as xe2x80x9cfilamentxe2x80x9d methods, employ one or more resistance heating units including heated wires or filaments, typically at temperatures of at least 2000xc2x0 C., to provide the high activation temperatures at which these disassociations take place. This method is known in the art as hot filament assisted chemical vapor deposition (xe2x80x9cHFCVDxe2x80x9d).
Many reaction and mass transport processes occur on the substrate surface. The substrate temperature is therefore crucial to optimize growth of the film.
The integrity of the resistance heating filaments is critical in achieving and maintaining uniform temperature during the deposition. If a heating filament sags, the temperature in the vicinity of the heating element differs, thereby creating thermal and species nonuniformity on the substrate. If filaments break frequently, maintenance is needed more often. The condition and position of heating elements is therefore a critical factor for reactor operation. Resistance heating filaments are delicate and are easily broken during operation of the reactor. They expand when heated from room temperature to an operating temperature of 2000xc2x0 C. and are thus subject to thermally-induced stresses that can break the filaments. Also, during carbon deposition, resistance heating filaments can react with carbon-bearing gases contacting the filaments and form a carbide, further lengthening and embrittling the filaments.
Horizontal filaments which are typically used in HFCVDs present a number of problems. The filaments severely deform and sag after they are first heated up so that uniform and repeatable film deposition become impracticable. As a result, intense maintenance work becomes necessary and makes it impractical to use most conventional HFCVD reactors for manufacturing purposes.
Because of tremendous thermal gradients present in a hot filament reactor, a large ceramic substrate plate breaks easily. For large area deposition, a uniform gas distribution is also necessary to achieve uniform deposition. It is thus important to be able to control the temperature of the substrate. Furthermore, graphitic carbon on non-substrate (i.e., the elements of the reactor) accumulates in HFCVD reactors. This accumulation requires frequent removal resulting in higher maintenance costs.
What is needed, therefore, is a device that can maintain a substantially constant force on the filaments to eliminate sagging of the filaments. Preferably, such a device would minimize maintenance and equipment set-up between cycles of operation. What is also needed is a device to control the gas flow. Preferably, such a device would also control the deposition of the carbon on the substrate and control the temperature of the substrate to prevent breakage of the substrate. Furthermore, a preferred device would also result in reduced maintenance costs by minimizing carbon accumulation.
The previously mentioned needs are fulfilled with the present invention. Accordingly, there is provided, in a first form, a disclosed CVD reactor which substantially reduces or eliminates the disadvantages and shortcomings associated with the prior art techniques.
To prevent filament sagging, the filaments in the present invention are arranged in the reactor vertically instead of the typical horizontal configuration. The filament assembly is configured such that the lower end of each filament can freely contract or extend vertically while constrained in all other directions.
The reactant gas is introduced into the reactor chamber through a gas dispersion system. The gas dispersion system, mounted within the reactor chamber, has a configuration to introduce the gas into three separate feeding zones. Each feeding zone has an independent control of the gas feeding rate. The extent of radial gas flow in the reactor, therefore, is controlled. Thus, the typical nonuniform distribution of gas is reduced by utilizing a three-zone feed gas distribution so that a uniform deposit of material can be achieved over the entire surface of the substrate.
A substrate support is mounted within the reactor chamber. During the operation of the reactor, the substrate support continually rotates 180 degrees back and forth. This constant rotation also helps ensure a uniform distribution of the carbon onto the surface of the substrate.
To prevent substrate breakage, there is also a feature to preheat the substrate before the actual deposition process is started. This preheating reduces the thermal gradient created by the filaments. There is also provided a means to cool the substrate holder when the reactor is fully operational. This cooling mechanism allows the user to independently control the temperature of the substrate.
Furthermore, the filament support structure and other relatively hot surfaces inside the reactor are water-cooled so that little, if any, graphitic carbon can accumulate on these surfaces. Thus, maintenance costs due to this accumulation are greatly reduced.